A vast majority of today's electronic systems require a high precision frequency source to operate. Applications include, among others, communication systems, microprocessors, and audio and video devices. As the internet of things market emerges, more and more battery-operated devices are required to interface with the infrastructure of the internet of things. For instance, wearables, mobile phones, medical devices and isolated sensors. A high precision low power frequency source is required to march these devices at their correct pace.
A widespread type of low power frequency source is the crystal oscillator. It consists of a piezoelectric crystal and an inverting amplifier, connected to each other to form a negative feedback loop. At a certain frequency range—a very narrow range—the feedback becomes positive and the circuit oscillates at a frequency within the range.
The piezoelectric crystal tends to mechanically vibrate at a certain frequency. The vibrations are accompanied by vibrations in the electrical voltage across the crystal nodes. The relation between these mechanical and electrical vibrations is bidirectional, that is, mechanical vibrations result in voltage vibrations and vice versa.
When a crystal is vibrating it has kinetic energy. The vibrations therefore decay as time goes by due to energy loss. To keep the crystal vibrating an energy source must introduce energy to the crystal. In a crystal oscillator, the amplifier is the energy source that keeps the crystal vibrating, and consequently, the oscillator's output voltage oscillating.
Right after powering up a crystal oscillator the crystal has no energy and does not vibrate. In practice, however, noise always exists in the oscillator, in particular a noise voltage between the crystal nodes. This noise is amplified by the feedback loop forming the oscillator at a frequency at which the feedback is positive. The crystal vibration and oscillator's output oscillation become stronger and stronger until the amplifier reaches saturation and it cannot add energy to the oscillator. From that point on the oscillator continues to oscillate in its steady state.
The aforementioned process taking place at power up is called the oscillator's start-up. It consists of energy build up by means of a positive feedback mechanism, a behaviour usually referred to as regeneration. As regeneration processes are slow in nature, it may take a long time—up to several seconds—for a crystal oscillator to start-up. Furthermore, since many crystal oscillators operate under low power conditions, the amplifier cannot introduce a lot of energy to the crystal in a short time. This increases the start-up time even more.
Shortening start-up time is vital to achieve a reasonable power up or wake up time of devices. It can be done by inserting a lot of energy to the crystal during start-up—a process that requires high power consumption—and then switching to normal operating mode for better power efficiency. Moreover, in low energy devices, e.g. battery-operated devices, where power efficiency is critical, start-up time should be as short as possible to minimise battery drain by the power consuming start-up process.